The third shell of an atom can have a maximum of ______ electrons. A. 13 B. 15 C. 18 D. 21

The correct answer is C. 18.

Each shell can hold a maximum of 2n^2 electrons, where n is the shell number. The third shell is n=3, so it can hold a maximum of 2*3^2 = 18 electrons.

Option A is incorrect because it is the number of electrons in the second shell.

Option B is incorrect because it is the number of electrons in the third shell of a lithium atom.

Option D is incorrect because it is the number of electrons in the fourth shell.